One known example of this type of lighting device is a so-called “LED in glass” device. An example is shown in FIG. 1. Typically a glass plate is used, with a transparent conductive coating (for example ITO) forming electrodes. The conductive coating is patterned in order to make the electrodes that are connected to a semiconductor LED device. The assembly is completed by laminating the glass, with the LEDs 4 inside a thermoplastic layer (for example polyvinyl butyral, PVB) or an organic resin layer.
FIG. 2 shows the known LED in glass structure in cross section. The device comprises glass plates 1 and 2. Between the glass plates are (semi-) transparent electrodes 3a and 3b (for example formed using ITO or thin conductive wires), and an LED 4 connected to the transparent electrodes 3a and 3b. A layer of electrically insulating material 5 is provided between glass plates 1 and 2 (conventionally PVB or UV resin).
Applications of this type of device are shelves, showcases, facades, office partitions, wall cladding, and decorative lighting. The lighting device can be used for illumination of other objects, for display of an image, or simply for decorative purposes.
Top emitting LEDs can be used, and these have a spot light source output. Side emitting LEDs can also be used, the light output then being coupled to the output from scattering centres in the resin.
To make the spot output more uniform, it is known to apply a reflector or other light shield over the LED, to act as a mask for reducing the local light output intensity. This adds complexity to the manufacture of the device so that there remain difficulties in providing a desired output from the discrete light sources, in terms of illumination direction and illumination spot size, and in a way which does not significantly increase the manufacturing cost.
There is also a desire to create luminescent sheets, preferably thin, with significant flexibility. This enables a wider variety of uses for the product and can also save on expensive moulding procedures in the manufacturing process. An optically transparent material is required with flexibility at the desired thickness of around 1-10 mm.
One example is to place LEDs within a wire grid that is embedded in silicone or other polymer. This design replaces costly Printed Circuit Boards PCBs or glass substrate arrangements. The wire grid, however, does not allow for a small tolerance in the alignment of the LED devices, by virtue of its non-rigid nature.
In order to mask the LED hot spots for LEDs formed in such a grid structure, optical structures (which may simply comprise reflective white paint) can be provided over the LED grid as mentioned above.
An alternative approach is to embed light scattering particles into the polymer structure (which acts as a light guide). The shape or concentration, and the absorption, reflection and transmission properties of these particles, influences how they alter the light output characteristics.
The main drawback of these techniques is that the position of the particles or the optical structures and their scattering properties are determined in the manufacturing process of the system and do not take into account the position of the light sources. This requires a costly alignment of the optics with respect to the light sources. The required optical structures or scattering particles that are used to mask the hot spots require a level of alignment in the range 10-100 micrometers with respect to the LED position. Thus, the accurate alignment of optical structures or particles with respect to the LEDs presents problems.
Prior art document WO2010/129815A2 describes light guide system in which light sources are embedded within the body of a light guide, which light guide comprises light deflecting particles. Said particles may be small bubbles or regions with a different refractive index. The concentration of the light deflecting particles can be varied based on the location of the embedded light sources in order to obtain a desired light emanation pattern. The prior art document is silent about methods to realise such concentration variations.
The present invention aims at providing a light output device of the type described in the previous paragraph, which can produced in a practical manner.